The invention relates to an acoustic transducer system including an electromechanical transducer, a circular flexural vibrating plate coupled to the electromechanical transducer and so configured that it is stimulated to higher order flexural vibration at the system operating frequency, at which nodal lines form on the flexural vibrating plate between which first and second antinodal zones are located oscillating alternatingly opposite in phase so that the flexural vibrating plate emits sound waves into a transmission medium bordering one side of the flexural vibrating plate or is stimulated to flexural vibration by sound waves arriving via the transmission medium, and including means for influencing the sound radiation by the flexural vibrating plate.
Acoustic transducer systems of this kind are used more particularly as sound transmitters and/or sound receivers for echo ranging wherein the travel time of a sound wave emitted by a sound transmitter to a reflecting object and the travel time of the echo sound wave reflected by the object back to the sound receiver is measured. For the known speed of sound the travel time is a measure of the distance to be measured. The frequency of the sound wave may be in the audible range or in the ultrasonic range. In most cases ranging is done in accordance with the pulse delay method in which a short sound pulse is emitted and the echo pulse reflected by the object is detected. In this case the same acoustic transducer system may be used alternatingly as the sound transmitter and sound receiver.
One broad field of application of this sonic ranging technique is level sensing. For this purpose the acoustic transducer system is located above the material to be sensed, above the highest level of the material anticipated, so that it radiates a sound wave downwards onto the material and receives the sound wave reflected upwards from the surface of the material. The measured travel time of the sound wave then indicates the distance of the material surface from the acoustic transducer system, and for a known mounting level of the acoustic transducer system the level to be sensed may then be computed.
For sonic ranging over long distances high-performance acoustic transducer systems having a good efficiency are needed so that the intensity of the received echo signal is still sufficient for analysis. The efficiency depends mainly on two factors:
1. on how well the acoustic transducer system is adapted to the impedance of the transmission medium; PA1 2. on the directivity of the acoustic transducer system in transmitting and receiving sound waves.
The flexural vibrating plates used in known acoustic transducer systems serve for impedance matching. In level sensing the transmission medium for the sound waves is gaseous, e.g. air, this also applying to many other fields of application. Conventional electromechanical transducers, such as piezoelectric transducers, magnetostrictive transducers, etc. have as a rule an acoustic impedance which is very different to that of air or other gaseous transmission media. This is why they serve in known acoustic transducer systems merely for stimulating the large surface area flexural vibrating plates forming the actual sound radiators or sound receivers and result in good impedance matching to air or other gaseous transmission media.
As regards the desired directivity large surface area flexural vibrating plates would appear to be likewise of advantage since it is known that pencilling a radiation lobe is the narrower the greater the extension of the radiation surface area in relation to the wavelength. This is hampered, however, by the problem that the antinodal zones oscillating alternatingly opposite in phase emit sound waves opposite in phase causing interference with each other in the case of acoustic transducer systems incorporating a flexural vibrating plate exhibiting higher order flexural vibration.
To avoid this unfavorable radiation pattern it is known from "The Journal of the Acoustical Society of America, Vol. 51, No. 3 (Part 2), pages 953 to 959, to configure the portions of the flexural vibrating plate corresponding to the antinodal zones alternatingly differing in thickness. This difference in thickness is so dimensioned that the sound waves emitted by the thicker portions receive a phase rotation through 180.degree.. The sound waves radiated from all antinodal zones are then in phase so that the radiation pattern features a marked radiation maximum in the axial direction in the form of a pencilled lobe. Producing such a flexural vibrating plate is, however, complicated and expensive. Furthermore, the acoustic transducer system equipped with such a flexural vibrating plate has a very narrow band since phase rotation through 180.degree. occurs only for a highly specific frequency as dictated by the structure of the flexural vibrating plate, this being the reason why it is not suitable for pulsed operation.
In an acoustic transducer system known from European Patent EP 0 039 986 the portions of the flexural vibrating plate corresponding to the alternating antinodal zones are likewise configured so that the sound waves generated by every second antinodal zone receive a phase rotation through 180.degree., resulting in the sound waves emitted from all antinodal zones being substantially in phase. For this purpose a low-loss acoustic propagation material is applied to the corresponding portions of the emitting surface area of the flexural vibrating plate in such a thickness that the desired phase rotation is achieved, closed cell expanded plastics materials or non-expanded elastomers being proposed as the low-loss acoustic propagation material used for this purpose. This material needs to be cut out corresponding to the shape of the antinodal zones and bonded to the flexural vibrating plate, thus resulting in problems when the acoustic transducer system is exposed in operation to mechanical stresses or chemical influences as is particularly the case in level sensing. The bonded plastics parts are susceptible to damage and are only weakly resistant to many chemically aggressive media. Furthermore, they increase the risk of encrustations of dusty, powdery or tacky material, this impairing reliable functioning.
In an acoustic transducer system known from German patent 36 02 351 a sonic beam shaper is provided to influence the sound emitted, comprising sound wave barriers which are impervious for sound waves, located spaced away from the flexural vibrating plate and acoustically decoupled therefrom in front of antinodal zones oscillating in phase relative to each other, whilst portions which are pervious for sound waves are located in front of the remaining antinodal zones oscillating opposite in phase relative to the former antinodal zones. The sonic beam shaper has the effect that only in-phase sound waves are radiated by the flexural vibrating plate whilst the sound waves opposite in phase thereto are suppressed by the sound wave barriers.